Do not return an empty map with secrets

This is causing some issues in secret agent from plasma-nm, because it
always tries to store VPN secrets to KWallet even when the map is empty
and when it loads this empty map later it overwrites the secrets we get
from NetworkManager in GetSecrets method and thus when plasma-nm displays
the auth dialog it doesn't have necessary secrets available.

BUG:339296
CCBUG:309931
CCBUG:334474